Public health advocates have long set their sights on wiping out polio worldwide, but recent resurgences of the pernicious disease raise questions about its future eradication.

Several months ago a wild strain of the virus surfaced in a sewer system in Rahat in southern Israel, and now it has reportedly been detected throughout the country. Israel’s government this week launched a nationwide vaccination campaign, attempting to inoculate all children under nine years of age with oral polio vaccine (OPV), a form of the vaccine containing a live, weakened form of the virus. Most of these children were already vaccinated as babies with inactivated polio vaccine (IPV), otherwise known as the dead-virus vaccine. But people who were injected with IPV can still be healthy carriers of the disease and shed the virus in feces.

Scientific American spoke with Bruce Aylward, assistant director general for Polio, Emergencies and Country Collaboration at the World Health Organization, to find out more about the situation in Israel and how recent events there are affecting global efforts to wipe out the disease.

[An edited transcript of the interview follows.]

What is happening in Israel right now?
What we know is that there is widespread detection of a wild polio virus at a number of sites that we have sampled, going back three-plus months. This virus is very similar to a strain that was detected in December of last year in Egypt, in the sewage there. This original virus came from Pakistan. Whether it went into Egypt and then Israel or Israel and then Egypt or [whether it spread via] two separate importations—it is unclear.

The virus has only been found in sewage at this point. There have not been any clinical cases of this so far; no children have been paralyzed. In the past [Israel] has detected [polio] virus from surrounding countries and it has disappeared very quickly, but this time it is persisting for longer. The virus can’t live in the sewage itself and multiply. What we are seeing is persistence of [people excreting] the virus.

How high is Israel's vaccination coverage?
This is a country with quite high immunization coverage—about 94 percentage of coverage. It’s with the inactivated virus, the dead-vaccine virus that Dr. [Jonas] Salk made in the 1950s (versus the live vaccine coverage that [Albert] Sabin developed, which we mainly use in the vaccination program). Since the kids don’t have intestinal immunity, or not very much, the disease is managing to spread.

The reason the oral vaccination is used in the vaccine campaign is it provides intestinal immunity that is so crucial in stopping the person-to-person transmission spread in settings where you might have a high transmission rate of the virus—like in tropical areas or areas with suboptimal sanitation. For a long time in developed countries Sabin’s vaccine was the vaccine of choice, but the drawback was one in a million times a child can get the disease and get paralyzed. It’s very rare, but it’s a risk.

As global progress was made on eradiation, many countries switched to the inactivated vaccine. One country that solely uses inactivated vaccine is Israel.

So the kids who were vaccinated as babies are protected but they can still be carriers?
With IPV you are protected, but you will still shed the virus. Your goal [with vaccination] is the person doesn’t get polio when you vaccinate and also they don’t spread it. With IPV you protect the individual but don’t do as much to protect the gut and protect the community. With OPV you get both protection of the individual and the community.

What's the main challenge to getting more people to take oral vaccine in a situation like this?
In countries where people are no longer using the oral vaccination, people are saying, “Why aren’t we using this vaccine now? Because it can cause paralysis, outbreaks, etcetera.”

The inactivated polio vaccine has no serious adverse events associated with it. The oral polio vaccine has extremely rare but real risks. The most common and predictable adverse event is Vaccine-Associated Paralytic Polio, or VAPP. The risk is less than one in a million and it’s mainly associated with the first dose of the vaccine. The [live virus] vaccine itself is not causing paralysis [with VAPP] since [the vaccine contains] a weakened form of the virus. It’s the vaccine virus replicating in the kid and reverting to virulence.

What have other countries done in a situation like Israel’s?
Other countries, like the Netherlands—IPV-using countries—when they suffered an outbreak due to importation of the virus, they used OPV to stop transmission of the disease. OPV remains the method of choice to rapidly stop an outbreak.

It’s Russian roulette to let a virus circulate in your country knowing you have susceptible kids. Even with 94 percent coverage [via the injected vaccine with the dead virus] you still have 6 percent of the population that can get the virus, and there are still risks among the 94 percent, too. There will be paralytic cases unless you stop transmission of this thing.

How does the disease spread via feces?
Think about the last time you were in a washroom and how many people did not wash their hands. As much as we would like to pretend sanitation is fabulous all over the world, we know that everyone’s personal hygiene is not perfect. Polio is one of those viruses where you only need an incredibly small infectious dose to get infected. If the virus is circulating in an area, there’s a high probability that you could get exposed. It spreads from fecal-oral transmission.

How concerned are you about transmission beyond Israel?
Any persistent transmission anywhere in the world right now is alarming and should be treated like a health emergency to the country, but also to the surrounding areas. The virus is a silent hitchhiker that moves in the guts of people that have been vaccinated. IPV-vaccinated people could be carrying the virus without even knowing it. It’s when you see this persistence—as we’re seeing now—and expansion that is concerning. In Israel, if they don’t stop it and it does spread, there are other countries around it having problems with their immunization campaigns we don’t want to see get reinfected.

Polio recently reemerged in Somalia. How does this strain differ from the Somalia strain?
That’s an African strain. Both are serotype 1 viruses. There are three types of polio. This is the first year in history we have only seen one type of polio. The last type 3 we saw was in November of last year in Nigeria. We’re not seeing type 2. This is the first year we have seen six-plus months without a type 3 virus. The virus found in Israel originated in Pakistan, in south-central Asia. The Somalia virus came from the western African route that is mainly circulating in Nigeria.

What’s the status of the global effort to eradicate polio overall?
We are closer than we’ve ever been. We’re only dealing with one type of virus now and seeing less substrains of that virus. Within Pakistan and Nigeria and Afghanistan—the three parts of the world that never stopped seeing polio—[the viruses] are in the smallest parts of the country we’ve ever seen and the smallest number of cases we’ve seen. Although Israel has been reinfected, it has stopped [the disease] so many times before and will stop it again. Although Somalia has been reinfected—and there is already a big outbreak—it has been reinfected multiple times, too. The key [to beating back polio] is what is happening in Nigeria and Pakistan. Afghanistan is just seeing polio from right across the border in Pakistan.

Médecins San Frontières [Doctors Without Borders] recently pulled out of Somalia due to the violence there and risks for its workers. Does that hurt the country’s chances of tamping down its polio outbreak?
The big implication of MSF pulling out [has to do with distributing] broader health services. Its one of the few health providers providing basic services in certain parts of the country. In terms of polio eradication, if MSF or other groups are there, that will help, but with a couple of hours of training, anyone can give [oral] vaccines to the kids.

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